A polyester represented by a polyethylene terephthalate is excellent in mechanical properties, thermal properties, electrical properties, etc. and is thereby widely used for fibers and for molded products such as films, sheets and bottles for various applications and its demand is expanding. A polyester to be used for such applications is usually produced by subjecting a dicarboxylic acid and/or its ester-forming derivative and a diol to esterification and/or ester exchange reaction and then melt polycondensation reaction, and as the case requires, further to solid phase polycondensation.
A polyester after the melt polycondensation reaction is usually granulated to particles with a particle size of from about 1 mm to about several mm so as to be formed into a product or to be subjected to solid phase polycondensation. As a granulation method, usually, a method (hereinafter sometimes referred to as a “strand cutting method”) of discharging a melt polycondensed polyester from a die plate having a plurality of pores (die holes) into a gas phase to form the polyester into strands, and subsequently cutting the strands by a fixed blade and a rotary blade having a rotation axis substantially at a right angle to the discharge direction under cooling with water or after cooling with water, is excellent in view of production efficiency and is commonly conducted.
However, when a polyester having a relatively low intrinsic viscosity after the melt polycondensation reaction, for example, an intrinsic viscosity of at most about 0.3 dL/g, is to be granulated in order that the polyester is used for special purpose or to be subjected to solid phase polycondensation, the polyester can hardly be wound in the form of strands since it has a low melt viscosity, and the strands after cooled are fragile, and accordingly granulation by the above strand cutting method has been known to be very difficult. Therefore, as a method of granulating such a polyester having a low intrinsic viscosity, for example, Patent Documents 1 and 2 propose granulation method other than the strand cutting method.
Further, as an improved strand cutting method, Patent Document 3 discloses a method of extruding a large number of strands of a molten thermoplastic resin material from multiple die toward an inclined trough including a coolant stream to bring the strands into contact with water in the trough, carrying the strands downward by the stream to rapidly cool the stands and then cutting the strands into particles by a strand cutter. The document discloses a method wherein the linear velocity of the coolant stream flowing down the trough is higher than the linear velocity of the strands extruded from the die, and the edge of strands newly formed which have passed through the strand cutter connected to the edge of the tough directly with the strands are made to pass through an apparatus along a flow path similar to the flow path of preceding strands.
The above disclosed method is a method excellent for forming (pelletizing) a thermoplastic resin material as a common molding material into particles, but by this method, strands of a polyester low polymer having a very low melt viscosity will not stably run, and formation into particles is difficult. Further, particles to be subjected to solid phase polycondensation are required to have a small particle size so as to increase the solid phase polycondensation reaction rate, but with the technique disclosed, it is further difficult to obtain particles with a small particle size.
Patent Document 4 discloses a method of stretching a ribbon, gut or sheet of a polyester with a low degree of polymerization having an intrinsic viscosity of from 0.32 to 0.40 dL/g between rolls with a draw ratio less than 2 times, followed by cutting. In this method, as shown in FIGS. 1 and 2 of the Patent Document 4, the range of the proper draw ratio is very narrow at an intrinsic viscosity of at most 0.35, and this method can not be employed for practical use.
Further, Patent Document 5 discloses a method of forming a molten polymer containing a polyester as the main component to a molding material in the form of pellets, which comprises cooling molten polymer strands discharged from an outlet in the air for from 0.10 to 0.50 seconds and then bringing them into contact with cooling water and solidifying them to form pellets. However, by the method disclosed in the document, favorable pellets can not necessarily be obtained from a polyester having a low melt viscosity.
Further, Patent Document 6 discloses cylindrical particles made of a polyester having an inherent viscosity of from about 0.20 to about 0.45 dL/g and their production method. The requirement of the method disclosed in the above document is that molten strands are made to pass in a narrow air gap i.e. an air gap less than about 4 inch (less than about 0.10 m) before rapidly cooled by a cooling medium to obtain cylindrical particles. However, in this method, it is difficult to stably granulate a low viscosity polyester, and particularly, it is difficult to efficiently produce polyester particles with a small particle size having a substantially cylindroid shape, suitable for solid phase polycondensation at a high speed in the present invention, from a low viscosity polyester.
As described above, no specific method of efficiently forming a polyester with a low degree of polymerization into particles has been known yet.
Patent Document 1: JP-A-51-066346
Patent Document 2: JP-A-10-512510
Patent Document 3: JP-B-55-016806
Patent Document 4: JP-B-50-024359
Patent Document 5: Japanese Patent No. 2993369
Patent Document 6: WO2004/035284